I run several sites from a yahoo web hosting account. When people try to sign up on one of my wordpress blogs it tries to email them from their own email address instead of a webmaster address for the site. This causes yahoo web hosting to reject the emails from being sent if the user isn't using an email registered with my sites domain.
Any forums or other software I use works fine, just not word press. I believe this could email could easily be setup so it emails everyone from one address which the owner of the site would setup.
The only way anyone running wordpress on a yahoo web hosting account can have someone register for their site is to manually register them and create their password for them.